The present invention is particularly applicable for use in connection with applying a thick layer of photoresist for use in connection with a print head and, therefore, the invention will be described with particular reference to a thick photoresist layer used as a component of a print head. However, the invention has much broader applications and may be used in connection with many other products.
For example, the invention of this application is particularly useful in connection with products which can effectively use a photoresist if the photoresist can be applied in a thick and uniform manner. This can include x-ray conversion screens wherein an even top surface of a photo polymer is required because it must be brought into close contact with the surface of an image sensor array. Further, the photo polymer may be bonded to the image sensor array which necessitates a smooth uniform surface for good adhesion. Another application involves microfluidic circuits wherein the micro-channels or micro-reservoirs, which are patterned into the photoresist, are typically being sealed by bonding a cover plate, such as a glass plate, to the photoresist. Accordingly, an even surface on the photoresist layer is required. An uneven surface would require an excess amount of adhesive which would in turn clog the micro-channels or micro-cavities of the device. Even yet another potential application for the invention of this application relates to flat panel displays wherein a photoresist can be used as a space material in the flat panel displays such as plasma displays. A layer thickness of greater than 1 mm can be required which must have good uniformity. Uneven surface topography has to be avoided because a plate, such as a glass plate, is bonded to the photoresist in order to seal the display. Yet a further application for the invention of this application relates to applying a photoresist coating onto a substrate which has high topography such as in connection with micro-electro-mechanical systems (MEMS). The method according to the present invention will allow a smooth topography photoresist layer to be applied to the MEMS even though the substrate is uneven.
It is, of course, well known that a photoresist can be used in connection with a mask to produce a desired geometric shape on a substrate. In this respect, the geometric shape is transferred to a substrate utilizing a lithographic process. This process includes first applying a photoresist onto the substrate. In order to apply the photoresist, it must be in a liquid form which in prior art methods necessitated a solvent to be used in connection with the photoresist material. The solvent in the photoresist produces the necessary flow qualities and allows the photoresist to flow onto the substrate such that a relatively smooth topography is achieved. Next, the solvent must be evaporated from the photoresist material. While the solvent will naturally evaporate, heat is used to speed the process. Once a sufficient amount of solvent is removed, the photoresist solidifies upon cooling to room temperature. It should be noted that although most photoresists used in microelectronics contain solvents, photosensitive polymers exist which do not contain any solvents but which are in liquid form prior to light exposure (for example photosensitive adhesives from DYMAX corporation or standard resins used in stereolithography). The photoresist is a polymer that is radiation-sensitive thereby reacting to exposure to ultraviolet light, electron beams, x-rays or ion beams, for example. Accordingly, a mask is placed over the photoresist which corresponds to the desired geometric shape. The mask blocks the ultraviolet (UV) rays or light in the desired location so that the photoresist material is only selectively exposed. Once the mask is in place, the ultraviolet light is applied and subsequently, a solvent is used to wash away the undesired portions of the photoresist leaving the desired geometric shape on the substrate.
The mask used in the exposure process will depend on the desired geometric shape and whether the photoresist is a negative or a positive photoresist. In this respect, the mask used on a negative photoresist will be configured to allow the (UV) light to penetrate only the areas of the photoresist which are to remain after the process is complete. With this type of photoresist, the UV light will cause cross-linking in the photoresist. The portion of the photoresist which is not exposed to the UV light will not be cross-linked and, therefore, will be dissolved by the solvent applied after the exposure process. The opposite is true for positive photoresist materials. The mask will be configured to allow the UV light to penetrate only the portions of the photoresist which are to be dissolved away. It should be noted that the invention of this application can be practiced in connection with either positive or negative photoresists.
Prior art methods of applying a photoresist to a substrate have many problems associated with producing a uniform topography in the resulting photoresist layer. One of the problems is that uneven topography of the photoresist layer worsens as the thickness of the layer increases. As is shown in Chun U.S. Pat. No. 6,191,053, prior art methods used to apply a photoresist to a substrate include spinning the photoresist onto the substrate. The spinning process requires a solvent to be used in connection with the photoresist to produce the necessary flow from the ejector nozzle of the spinning device onto the substrate. Referring to FIG. 11 of Chun, the photoresist nozzle is spaced from the substrate and includes a passage for dispensing the photoresist. Once the photoresist exits the nozzle, it flows onto the substrate producing a thickness H which is less than the space between the nozzle and the substrate. While the spinning process can effectively apply a photoresist to a substrate, the thickness is a function of the flow characteristics of the photoresist and is difficult to control for thick resist layers. The spinning process can only produce an accurate thickness for thin layers. Therefore, several passes are necessary to produce a thick photoresist layer. The multiple passes disadvantageously increase the time necessary to coat the substrate and also produce a photoresist layer with an uneven topography. Furthermore, after each application, the edge bead of the photoresist may have to be removed which is an additional step and is wasteful of the expensive photoresist material. In order to produce a smooth topography, a costly polishing step is necessary which is also time-consuming.
For a high nozzle density (greater than 600 dpi resolution) printer it has been found that channel-shaped ink ejectors can be patterned using photolithography in connection with a thick layer of epoxy photoresist. The layer thickness involved is between 100 and 1,000 microns. However, the height of the channels must be very uniform over a large area since an attachment plate for the print head must be bonded to the top surfaces of the channels. Height variations in the channels require a thick glue layer for bonding and sealing the attachment plate to the photoresist channels. As can be appreciated, utilizing a thick glue layer could cause clogging of the channels. Furthermore, in order to obtain the thickness necessary to produce the channels, several thin layers would need to be produced using the spin coating method. After each layer, the edge bead would need to be removed. Again, as stated above, a polishing step would also need to be performed to produce the necessary uniform ink ejectors.
The uneven photoresist topography is not only a result of the prior art application methods, the prior art baking process can further impact uniformity of the photoresist if the hot plate is not perfectly level. This is the result of the photoresist flowing during the long soft bake process which can take several hours.